In light microscopy, cover glasses or laboratory dishes and the like are frequently used to cover, enclose, or protect the specimen. Microscope objective lenses intended to be used with cover glasses and the like normally specify a specific refractive index and thickness of the cover glass. Consequently, whenever the thickness or refractive index of the cover glass or laboratory dish is not as specified for the objective lens, spherical aberrations can arise that degrade imaging performance of the objective lens and reduce contrast of the image.
Metallurgical microscopes and the like used to observe metal and semiconductor surfaces typically utilize objective lenses that are not designed to be used with a cover glass or other refractive body between the specimen and the objective lens. Thus, such objective lenses are designed without consideration of the thickness of a refractive body through which the lens would otherwise view the object. In actual practice, however, there are times when a glass plate or the like must be used to protect the surface of a semiconductor wafer or integrated circuit during microscopic examination. In such instances, the generation of spherical aberrations by the plate can make high-quality observations of the specimen impossible.
Immersion objective lenses adapted to be used with an immersion fluid such as water or oil are also adapted to be used with such fluids having a particular refractive index. If the refractive index of the immersion fluid changes, then unwanted spherical aberrations can arise that make high-quality observations impossible.
In certain conventional objective lenses intended for use with a refractive body situated between the objective lens and the specimen (e.g., cover glass or wall of a laboratory dish), an adjustable correcting lens (adjusted by turning a so-called "correcting ring") is conventionally employed to vary the internal spacing between lenses or lens groups within the objective lens. The amount by which the lens spacing is adjusted is a function of the amount of spherical aberration generated by the refractive body. The adjustment serves to offset changes in such spherical aberration as viewed by the observer. Microscope objective lenses of this sort are disclosed in Japanese laid-open patent document nos. Sho 60-205521 and Sho 60-247613.
Conventional adjustable objective lenses require that a considerable amount of air space be provided in front of (i.e., objectwise) and behind (i.e., imagewise) certain lens groups in the objective lens to provide a sufficient range of adjustment. Such space requirements significantly limit the degrees of freedom that can be exploited in the design of the objective lens, thus increasing the difficulty with which the performance of the lens can be optimized.
Another problem with conventional adjustable objective lenses is that the mechanical device used to move the movable lens(es) lenses had to be located in the objective lens. This undesirably increased the size and complexity of the objective lens.
With conventional objective lenses that are not designed to be used with an intervening cover glass or the like between the lens and the specimen, if a specimen were observed with a cover glass in place, spherical and other aberrations caused by the thickness of the cover glass reduce the contrast of the observed image, thus making high-quality observations impossible.